In the technique or uranium enrichment described in U.S. Pat. No. 3,772,519, and United States patent application Ser. No. 328,954, filed Feb. 2, 1973 and Ser. No. 469,407, filed May 13, 1974, all incorporated herein by reference, laser radiation is applied to a uranium vapor to create isotopically selective photoionization of isotopes of a desired, typically U-235, isotope without corresponding ionization of other isotope type particles. The technique illustrated there typically employs a plurality of energy steps in exciting the uranium particles from the ground energy state to the ionization continuum.
For employing this technique in production scale enrichment, it is advantageous to provide laser radiation for each energy step below ionization of sufficient intensity to create a condition wherein the illuminated particles are equally distributed among all of the reachable states. This condition is commonly referred to as saturation, the theoretical maximum percent excitation that can be achieved in a column of particles exposed to steady laser radiation. Since plural transitions may typically be employed to reach the ionization continuum, it is apparent that each of N energy states will be populated to an extent which is only 1/N of the population of the illuminated particles. If, for example, two steps having three energy states are used to reach the upper most level below ionization, there will be in the uppermost energy state below ionization at most only 1/3 of the total illuminated particles at any given moment. While the process of depletion of particles from this excited energy level by ionization will permit the population of particles to be continuously renewed, the steady state, relatively small percentage population of that energy level, will reduce the ionization cross-section which is typically already much smaller than the cross-section for the other transitions between excited energy states.